The overall objective of this project is to develop new wear resistant, nanostructured composite materials for use as direct and indirect placement restorative materials. To achieve this objective the following specific aims have been proposed: Specific Aim 1 of the program proposes to process and characterize several unique composite structures utilizing various combinations of inorganic and organic polymers. Several previously reported sol-gel processing techniques have been proposed as methods of producing novel monophasic and diphasic silicate, aluminate and phosphate nenoporous and colloidal filler particles. Based on our preliminary data, we hypothesize that nanostructured wear resistant dental composite materials could result from the combination of these unique particles with polymerizable monomers. This rather simple but unique combination of materials which result from a series of polymerization reactions, can lead to an intimate combination of inorganic and organic substances. The filler particles themselves consist of a microporous inorganic cross linked network structure which can be impregnated with polymerizable methacrylate monomers. The in situ polymerization of these materials result in nano scale interpenetrating inorganic/organic complex. The silicate/methacrylate combination will serve as the model by which the optimal particle parameters (size, shape, density) will be evaluated. This familiar compositional component combination put together in a unique microstructural manner is felt to have the greatest potential for immediate application. New component inorganic and organic phase combinations have been proposed which offer the potential to improve the chemical stability and or biocompatibility of the structure. Specific Aim 2 of this program is designed to assess the potential clinical performance of these unique nonostructured composite materials. The initial and principal focus of the evaluation will center on the assessment of wear resistance. The proposed in vitro tests utilize wither a two body microabrasion or a cyclic surface microstressing technique to assess wear. Current dental composite resins will be used as negative controls and human enamel as a positive control. The overall degradation processed involved in wear resistance as it relates to the chemical stability of the inorganic and organic polymer components will also be assessed. Materials which perform significantly better that current dental composites under at least one test condition are likely to result in composites which exhibit improved clinical performance and will be further evaluated. Other properties of significant importance which will be examined on the most promising materials include creep, fracture toughness and strength, elastic modulus, hardness, and fatigue. The sequence of work proposed is designed to systematically evaluate the relationship between the composite composition and microstructure and the ability of composite materials to resist wear. Completion of the proposed work is expected to lead to the development of new wear resistant dental composite materials.